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Tensions between Japan and China are high over a group of disputed islands

Japan controls the islands, but China claims they are part of its territory

Six Chinese maritime surveillance ships entered waters around a group of islands at the center of a heated territorial dispute between Tokyo and Beijing, ignoring warnings from the Japanese authorities.

The islands, situated in the East China Sea between Okinawa and Taiwan, are currently under Japanese control, but China claims they have been an "inherent" part of its territory "since ancient times."

The Chinese ships entered Japanese territorial waters despite warnings from the Japanese Coast Guard, said Shinichi Gega, a spokesman for Japan's 11th Regional Coast Guard Headquarters. Three of the ships have since left Japanese waters, he said.

The Japanese government will "take all possible measures to ensure security" around the islands, Prime Minister Yoshihiko Noda said Friday.

Two of the Chinese ships responded to a Japanese Coast Guard vessel's warning by reiterating China's territorial claim to the islands and saying they were carrying out patrol work, according to Gega. Japanese ships are continuing their own patrols of the area, he said.

The controversial Chinese move to begin patrols around the islands follows the Japanese government's purchase of several of the islands from a private Japanese owner earlier this week, a deal that China described as "illegal and invalid."